efi: Clear up misconceptions about a maximum variable name size

The UEFI specification does not make any mention of a maximum variable
name size, so the headers and implementation shouldn't claim that one
exists either.

Comments referring to this limit have been removed or rewritten, as this
is an implementation detail local to the Linux kernel.

Where appropriate, the magic value of 1024 has been replaced with
EFI_VAR_NAME_LEN, as this is used for the efi_variable struct
definition. This in itself does not change any behavior, but should
serve as points of interest when making future changes in the same area.

A related build-time check has been added to ensure that the special
512 byte sized buffer will not overflow with a potentially decreased
EFI_VAR_NAME_LEN.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>